1. Field of the Invention
This invention relates to the field of treatment of cardiac arrhythmia and, in particular, new compounds and associated methods useful in such treatments.
2. Summary of Related Art
Congestive heart failure (CHF) is a disease affecting approximately 2% of the population of the United States (Sami, M. H. [1991] J. Clin. Pharmacol. 31:1081). Despite advances in the diagnosis and treatment of CHF, the prognosis remains poor with a 5-year mortality rate higher than 50% from the time of diagnosis (McFate Smith, W. [1985] Am. J. Cardiol. 55:3A; McKee, P. A., W. P. Castelli, P. M. McNamara, W. B. Kannel [1971] N. Engl. J. Med. 285:1441). In patients with CHF, the rate of survival is lowest in those patients with severe depression of left ventricular function and patients who have frequent ventricular arrhythmias. Patients with ventricular arrhythmias and ischemic cardiomyopathy have an increased risk of sudden death. The presence of ventricular tachycardia in patients with severe CHF results in a three-fold increase in sudden death compared to those without tachycardia (Bigger, J. T., Jr. [1987] Circulation 75 (Supplement IV):28). Because of the high prevalence of sudden unexpected death in patients with CHF, there has been a growing interest in the prognostic significance of arrhythmias in these patients.
Several compounds have been used in the management of cardiac arrhythmias in patients with congestive heart failure. Unfortunately, anti-arrhythmic drug therapy has been disappointing. The efficacy of anti-arrhythmic drugs markedly decreases as left ventricular function declines, such that only a small fraction of patients with CHF are responsive to anti-arrhythmic therapy. No anti-arrhythmic drug has prevented sudden death in patients with CHF. There is even a question of increased mortality associated with certain anti-arrhythmic drugs (the CAST investigators [1989] N. Engl. J. Med. 321:406).
Scientists define tachycardia and ventricular fibrillation as being of multiple nature. It now seems clear, and is accepted in the art, that re-entry is the underlying mechanism to most sustained arrhythmias. Prolonging ventricular repolarization as a means of preventing ventricular arrhythmias has consequently received renewed attention. This points to Class-III agents as drugs of choice in the treatment of arrhythmias. A Class-III agent, as referred to herein, is an agent which is classified as such in the Vaughan-Williams classification of anti-arrhythmic drugs. A Class-III agent exerts its primary anti-arrhythmic activity by prolonging cardiac action potential duration (APD), and thereby the effective refractory period (ERP), with no effect on conduction. These electrophysiological changes, which are brought about by blockade of cardiac potassium channels, are well known in the art. Because the blockade of cardiac potassium channels is not associated with depression of the contractile function of the heart, Class-III agents are particularly attractive for use in patients with CHF. Unfortunately, the existing Class-III agents are limited in their utility by additional pharmacological activities, lack of good oral bioavailability, or a poor toxicity profile. The only two Class III agents currently marketed are bretylium (i.v. only) and amiodarone (i.v. and p.o.).
Amiodarone is an anti-arrhythmic agent having vasodilator properties that may benefit patients with severe heart failure. Amiodarone has been shown to improve survival of post-myocardial infarction patients with asymptomatic high-grade ventricular arrhythmias, and it proved efficacious in patients resistant to other anti-arrhythmic drugs without impairing left ventricular function. Cardioprotective agents and methods which employ amiodarone in synergistic combination with vasodilators and beta blockers have been described for use in patients with coronary insufficiency (U.S. Pat. No. 5,175,187). Amiodarone has also been described for reducing arrhythmias associated with CHF as used in combination with antihypertensive agents, e.g., (S)-1-[6-amino-2-[[hydroxy(4-phenylbutyl)phosphinyl]oxy]-L-proline (U.S. Pat. No. 4,962,095) and zofenopril (U.S. Pat. No. 4,931,464). However, amiodarone is a difficult drug to manage because of its numerous side effects, some of which are serious.
The most serious long-term toxicity of amiodarone derives from its kinetics of distribution and elimination. It is absorbed slowly, with a low bioavailability and relatively long half-life. These characteristics have clinically important consequences, including the necessity of giving loading doses, a delay in the achievement of full anti-arrhythmic effects, and a protracted period of elimination of the drug after its administration has been discontinued.
Amiodarone also can interact negatively with numerous drugs including aprindine, digoxin, flecainide, phenytoin, procainamide, quinidine, and warfarin. It also has pharmacodynamic interactions with catecholamines, diltiazem, propranolol, and quinidine, resulting in alpha- and beta-antagonism, sinus arrest and hypotension, bradycardia and sinus arrest, and torsades de pointes and ventricular tachycardias, respectively. There is also evidence that amiodarone depresses vitamin K-dependent clotting factors, thereby enhancing the anticoagulant effect of warfarin.
Numerous adverse effects limit the clinical applicability of amiodarone. Important side effects can occur including corneal microdeposits, hyperthyroidism, hypothyroidism, hepatic dysfunction, pulmonary alveolitis, photosensitivity, dermatitis, bluish discoloration, and peripheral neuropathy.
Compounds of structural formula I:
and pharmaceutically acceptable salts thereof, wherein m is 0 to 4, n is 0 or 1, X1 and X2 are H, lower alkyl, or halogen, preferably iodine, R1 and R2 are lower alkyl, and R3 is optionally substituted alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl. Preferably R3 is (R)-2-butyl, (S)-2-butyl, (R)-3-methyl-2-butyl, or (S)-3-methyl-2-butyl.
The preferred compounds have the following formulas
and include pharmaceutically acceptable salts thereof. Particularly preferred salts are citrate, (L)-tartrate, (D)-tartrate, fumarate, maleate. These compounds are useful for treating mammals, including humans, suffering from ventricular and supraventricular arrhythmias, including atrial fibrillation.
These compounds, when administered to a mammal, including a human, give rise to metabolites that are useful for drug monitoring and that have electrophysiological properties of their own.
There is no Class-III agent presently marketed that can be used safely in patients with CHF. The cardiovascular drug market is the largest in any field of drug research, and an effective and safe Class-III anti-arrhythmic agent useful in patients with CHF is expected to be of substantial benefit. Therefore, a drug which could successfully improve the prognosis of CHF patients, but with a safety profile much improved over that of amiodarone, would be extremely useful and desired. Various analogs of amiodarone have been previously described (U.S. Pat. Nos. 6,710,070; 6,683,195; 6,372,783; 6,362,223; 6,316,487; 6,130,240; 5,849,788; 5,440,054; and 5,364,880). The subject invention adds to this arsenal of compounds.